"I read somewhere that half of all houses built since the war are on flood plains. The problem can often be that flood defenses are built to protect these new builds, but these just offset the impact of the flood upsteam or downstream. So older houses that were fine in the past suddenly find themselves underwater."
Don't know about the numbers but there's definately truth in this, some areas have essentially been turned into flood zones because of artificial changes to the watercourse further downstream. It's not so different to coastal defences, change the system in one place and something unexpected might happen somewhere else.
I'm a wee bit sympathetic to people in this situation, I think there's a reasonable assumption that if someone's built a house, it shouldn't be somewhere idiotic, and if you get a survey done on the building beforehand and it says "Roof's a bit iffy… Could do with double glazing…" but doesn't say "Oh and it's in a lake for 2 months a year" then that's an issue too.
And sometimes it's not at all obvious what's going to flood and what won't, you can be a long way from a watercourse and still be the lowest point, but if there's sections of higher ground between you and the river or a big impressive flood defence, then it's easy to see how people get caught out.